Existing non-contact tonometers measure IOP by activating a pump mechanism to fire an air pulse at the cornea to flatten or “applanate” a predetermined area of the cornea, detecting corneal applanation caused by the air pulse and a plenum pressure of the pump mechanism, and correlating the plenum pressure at the moment of applanation with IOP. In older instruments, the time elapsed to achieve applanation was correlated to IOP as an indirect representation of plenum pressure based on a linearly increasing pressure profile in the plenum. In present day instruments, a pressure sensor is mounted in the plenum for providing a signal proportional to the plenum pressure. Regardless of whether elapsed time or a signal from a pressure sensor is obtained, it is necessary to correlate the obtained quantity to IOP such that the instrument provides a meaningful measurement value of IOP as output. Thus, non-contact tonometers must be calibrated periodically to ensure that the correlation function used by the particular instrument yields IOP results that are substantially in agreement with an established standard of IOP measurement.
Traditionally, the Goldmann applanation tonometer (GAT), which measures IOP by directly contacting the cornea to applanate an area of the cornea, has been used as a standard for IOP measurement. Accordingly, initial calibration of a non-contact tonometer has been carried out by way of a clinical trial involving a large number of human eyes. During the clinical trial, each eye is measured with both GAT and the subject non-contact tonometer, and the parameters of a correlation function of the subject non-contact tonometer are adjusted to provide a best fit to the GAT results.
Conducting clinical trials is time consuming and expensive, and therefore clinical calibration might be conducted with respect to a “master” non-contact tonometer, and the master non-contact tonometer is then used as a calibration standard. It is known to measure “IOP” of a set of precision-manufactured rubber eyes designed and tested to applanate at predetermined pressures as a calibration gauge to avoid a clinical trial involving human eyes. Rubber eyes develop folds during testing and tend to be a poor simulation of a real eye. Moreover, rubber eyes are expensive and difficult to manufacture because very tight tolerances are necessary. Finally, the rubber material ages or can be damaged, so that a set of rubber eyes must be constantly recalibrated to maintain reliability.
The Physikalisch-Technische Bundesanstalt (PTB) of Germany developed a calibration tool for non-contact tonometers that employs a mirror and lever system, wherein the tonometer air pulse is directed at a mirror mounted on a lever to angularly displace the lever about a pivot axis. A working version of the tool incorporates a complex assembly of precision moving parts and is available at a cost of close to $30,000.00. Japanese Patent No. 11-225974 describes another mechanical calibration tool generally similar in concept to the PTB calibration tool in that it comprises a mirror mounted for measurable deflection by a tonometer air pulse.
The tonometer calibration devices and methods mentioned above are delicate, expensive, unstable, and/or difficult to use, and they cannot be traced to an absolute pressure standard such as that provided by a water column or precision pressure calibrator.
Against this background, U.S. Pat. No. 6,679,842 issued Jan. 20, 2004 to Luce discloses a tonometer calibration tool mountable in front of an air discharge passage of a non-contact tonometer. The calibration tool includes “an electronic eye” having a pressure sensor for receiving the air pulse and an infra-red emitter connected to the pressure sensor for providing a pseudo-applanation event, such as an infra-red pulse, when the pressure sensor signal reaches a predetermined level. The pulse acts as a pseudo-applanation event because in actual operation, the tonometer detects a peak level of light reflected by a flattened cornea to determine corneal applanation. In this way, the tonometer is induced to measure an applanation event in the absence of an eye. By adjusting the predetermined pressure level to known standards, calibration of the tonometer is possible. The entire disclosure of U.S. Pat. No. 6,679,842 is incorporated herein by reference. The '842 patent and the present invention share common ownership.
While the calibration tool of the '842 patent has greatly simplified calibration procedures, it has the drawback that it is time consuming to set-up. Specifically, the calibration tool of the '842 patent mounts on a fixed external housing of the tonometer, and a movable measurement head of the tonometer must then be aligned in X, Y, and Z dimensions relative to the calibration tool to bring the axis of the fluid discharge passage into alignment with the pressure sensor of the calibration tool and locate an exit end of the discharge passage at a predetermined working distance from the pressure sensor. In this respect, using the calibration tool of the '842 patent is similar to measuring an eye of a patient because the measurement head of the tonometer must first be moved into proper alignment before the air pulse is fired. To carry out this task, the calibration tool of the '842 patent includes a glass sphere 22 by which an opto-electronic alignment system of the tonometer can achieve three-dimensional alignment using light reflected by the glass sphere in the same way the alignment system would align to an eye using light reflected by the cornea. A mirror 26 on the calibration tool may be aligned on test axis in place of glass sphere 22 using a precision slide mechanism to allow for tilt and swivel adjustments. Once these preliminary alignment steps have been completed, the precision slide mechanism is used to insert the pressure sensor on the test axis in place of the mirror, and calibration measurements can be made. The preliminary alignment steps are time consuming, and they require additional parts such as the glass sphere, mirror, and precision slide mechanism that add size and cost to the calibration tool. If proper alignment is not carried out, calibration accuracy may suffer.
What is needed is a tonometer calibration tool that is smaller, less expensive, and easier to use.